Abstract:
The stress-strain distribution in flexural compression has been at the forefront of investigation ever since the 20th century. The original formulation of the flexural stress-strain distribution and the subsequent development of the stress block parameters, were based on specimens with a 127 x 203 mm (4 x 8 in.) cross section. The design of reinforced concrete flexural and flexural compression members at the Ultimate Limit State is based on the equilibrium of forces and moments obtained by using these stress block parameters. The calculation procedure entails the determination of the neutral axis depth, which varies depending on the magnitude of the applied action and the section dimensions. If the load action is small, the internal bending moment can be equilibrated with a reduced neutral axis depth, however, current design models do not consider the influence of a reduction in neutral axis depth (specimen size) on the stress block parameters, possibly resulting in an underestimation of the flexural compression capacity. This study aimed to evaluate the influence of specimen size and compressive strength on the stress block parameters of concrete by testing twenty-seven plain concrete specimens in flexural compression. Nine specimens were tested for each specimen size (50 mm, 100 mm, and 200 mm), three for each of the cylinder target strengths of 40 MPa, 65 MPa, and 80 MPa. The stress block parameters, obtained from the stress-strain curves, were compared to the data obtained by previous researchers, and the influence of specimen size on the stress block parameters evaluated for the different concrete strengths. Along with the size effect in flexural compression, the size effect for cubes and cylinders were also evaluated, and the associated cylinder strength used to eliminate the size effect of the stress block parameters. A comparison of the error between the predicted Moment-Axial force (M-N) interaction diagram, obtained by using the BS 8110-1 (1997), SANS 0100-1 (2000), ACI-318 (2014), and EN 1992-1-1 (2004) codes, and the actual M-N interaction diagram, obtained from the experimental points, were made, and conclusions regarding their applicability for the design of concrete containing South African materials drawn. Lastly, the flexural stress-strain behaviour was modelled, and a comparison made between the calculated and actual stress block parameters.
